1. Field of Invention
This invention relates to the method for the manufacture of structural foam articles having a predetermined skin thickness and a foam interior from the same basic resin compound.
2. Description of Prior Art
The present method of manufacturing structural foamed articles consist chiefly of two type of methods and apparatus. In one method as set out in the Angell U.S. Pat. No. 3,268,636 a method is described wherein a blowing agent is mixed with a plastic resin and then extruded into an accumulator until the proper amount is collected and kept at a temperature and pressure to prevent foaming. A valve is then transferred which now connects the accumulator to a mold wherein the plastic resin is forced into the mold where it will foam and expand forming a cell-like structure. The external skin of the article is chilled faster than the inside and therefore will form a continuous surface without holes or blemishes. However, with this method it is very difficult to control the skin thickness since the whole method relies on the mixing of the foaming agent, the foaming action of the plastic mixture, the temperature and pressure of the accumulator and mold structure, and its temperature gradient during cooling.
In the other method of injection molding foam products described in the Garner U.S. Pat. No. 3,599,290, two extruders are used wherein each contains a different type plastic resin. Here one extruder will inject a plastic of one type material into the mold, then a valve is transferred and a second type plastic, which will foam, is extruded into the mold to form the internal structure of the article. The mold halves are then opened a controlled amount and the foamed internal structure will fully develop. Here the skin thickness can be controlled because a fixed amount of that particular plastic was available within the mold from the first extruder and the foam will continue to expand giving the desired cell structure as the mold is allowed to open. Both quantities of each type plastic extruded is controlled by the timing of the extruders during the injection process so that the skin thickness and ultimate density of the article is controlled on a repeatable basis. Although not a problem but rather an expense in this second method is the use of two extruders for the skin and core of the article. My invention overcomes this drawback by requiring only one extruder.
In the former example the skin thickness was uncontrolled because when the accumulator was connected to the mold, the plastic flow was virtually uncontrolled and therefore the skin thickness variation and the ultimate core cell structure is a function of many variables which can change from shot to shot. Here my invention establishes a fixed quantity of solid material which will fix the minimum wall thickness as the foamed plastic is injected into the mold.